This invention relates to metallizable polyolefin films, and more particularly to the improvement of metal bonding properties of such films.
For some packaging films, the barrier properties are improved tremendously by vacuum depositing aluminum onto the surface of biaxially-oriented polypropylene films. Also, for decorating purposes metal deposition may be performed to give the film a reflective coating. Sometimes a lamination or other secondary process is performed which can damage the metal coating. Thus, a strong metal bond between the metal layer and the base or film layer is preferred. This preference extends more generally to other polypropylene films where greater metal bond strength improves the wear-life and quality of a deposited metal layer.
In films where metal coating is put directly onto a homopolymer polypropylene after surface treatment (such as corona treating (also known as corona discharge treating), flame treating, etc.) the metal bond is not noted to be very strong. However, often the physical and optical properties of a homopolymer polypropylene are more desirable to the overall objects of the film, necessitating against the use of a standard ethylene-polypropylene copolymer or ethylene-butene-polypropylene terpolymer, or other multiple polymer system known to have good bonding properties.
The polymers normally employed in the preparation of biaxially-oriented polypropylene films are isotactic polymers such as isotactic polypropylene, although on some occasions the use of syndiotactic polymers has been proposed. Isotactic polypropylene is one of a number of crystalline polymers which can be characterized in terms of the stereoregularity of the polymer chain. Various stereo-specific structural relationships denominated primarily in terms of syndiotacticity and isotacticity may be involved in the formation of stereoregular polymers for various monomers.
Isotactic polypropylene is conventionally used in the production of relatively thin films in which the polypropylene is heated and then extruded through dies and subjected to biaxial orientation by stressing the film in both a longitudinal direction (referred to as the machine direction) and in a transverse or lateral direction sometimes referred to as the xe2x80x9ctenterxe2x80x9d direction. The structure of isotactic polypropylene is characterized in terms of the methyl group attached to the tertiary carbon atoms of the successive propylene monomer units lying on the same side of the main chain of the polymer. That is, the methyl groups are characterized as being all above or below the polymer chain. Isotactic polypropylene can be illustrated by the following chemical formula: 
[Bolding for emphasis only]
Another way of describing the structure is through the use of NMR. Bovey""s NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each xe2x80x9cmxe2x80x9d representing a xe2x80x9cmesoxe2x80x9d dyad, or successive methyl groups on the same side of the plane of the polymer chain. As is known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
The isotactic polymers normally employed in the preparation of biaxially-oriented polypropylene films are usually those prepared through the use of conventional Ziegler-Natta catalysts of the type disclosed, for example, in U.S. Pat. Nos. 4,298,718 and 4,544,717, both to Myer et al. Thus, U.S. Pat. No. 5,573,723 to Peiffer et al discloses a process for producing biaxially-oriented polypropylene film based on an isotactic polypropylene homopolymer or propylene-ethylene copolymers. Other copolymers of propylene and alpha-olefins having from 4-8 carbon atoms also may be employed in the Peiffer process.
Catalysts employed in the polymerization of alpha-olefins may be characterized as supported catalysts or unsupported catalysts, sometimes referred to as homogeneous catalysts. Traditional supported catalysts are the so-called xe2x80x9cconventionalxe2x80x9d Ziegler-Natta catalysts, such as titanium tetrachloride supported on an active magnesium dichloride as disclosed, for example, in the aforementioned patents to Myer et al. A supported catalyst component, as disclosed in the Myer ""718 patent, includes titanium tetrachloride supported on an xe2x80x9cactivexe2x80x9d anhydrous magnesium dihalide, such as magnesium dichloride or magnesium dibromide. The supported catalyst component in Myer ""718 is employed in conjunction with a co-catalyst such as an alkylaluminum compound, for example, triethylaluminum (TEAL). The Myer ""717 patent discloses a similar compound which may also incorporate an electron donor compound which may take the form of various amines, phosphenes, esters, aldehydes, and alcohols. Metallocene catalysts are often employed as unsupported or homogeneous catalysts, although, as described below, they also may be employed in supported catalyst components.
Alternative types of catalysts that produce isotactic polyolefins are disclosed in U.S. Pat. Nos. 4,794,096 and 4,975,403. These patents disclose chiral, stereorigid metallocene catalysts that polymerize olefins to form isotactic polymers and are especially useful in the polymerization of highly isotactic polypropylene.
The present invention relates to metallized polyolefin film. The film includes a film layer formed of an ethylene-propylene copolymer where the ethylene is present in an amount of no more than about 1 weight percent and preferably between about 0.1 weight percent and about 0.7 weight percent. The film layer is surface treated (preferably corona treated) on at least one side (i.e., at least one surface), preferably to a level of at least about 48 dynes/cm as measured contemporaneously with treatment. The film layer is metallized after surface treatment with metal deposited on the treated surface of the film layer. The deposited metal layer has a thickness less than the film layer. The resulting film has a bond strength between the film layer and the metal layer which is at least 30 percent greater than the bond strength between the metal layer material and a correspondingly surface treated film layer formed of polypropylene homopolymer. The present invention further relates to a method for producing such a metallized film.